Scenario:
Place two or more dispensers (or droppers, or mix them, doesn't matter) in a horizontal row. Place blocks above them with repeaters pointing into each of those blocks. Now hook all of the repeaters up to a redstone source such as a button.
Expected behavior: Pressing the button causes all the dispensers to fire every time.
Actual behavior: The first time the button is pressed, all dispensers will fire (as expected). Subsequent times, some of them will fail to fire.
See http://youtu.be/PIdDBP00Jwg for a short video documenting the behavior.
NOTE:
Before you resolve this as a duplicate let me add that I have looked at MC-108 (the most likely match) as well as several other bugs and while the bugs are similar, to my mind they are NOT the same so please at least look closely at it first.
This is new behavior. I'm quite sure this was working at least in some of the snapshots leading up to 1.5. I am not sure exactly when it quit working but if I had to guess, I would suspect that it has something to do with the changes around having droppers and dispensers no longer react to nearby redstone updates.
Duplicate of MC-12464.